1. Field of the Invention
The present invention relates to the field of non-aqueous electrochemical cells and, more particularly, to a high energy density, non-aqueous electrochemical cell which safely withstands extensive cell voltage reversal.
2. Description of the Prior Art
Much work has been done in the field of high energy battery systems utilizing highly reactive anode materials such as alkali metals in combination with non-aqueous electrolytes. In these cells, the preferred alkali metal anode is lithium. The electrolyte normally includes a solute which is commonly a metal salt or complex metal salt of the metal anode material dissolved in a compatible non-aqueous solvent depolarizer. Examples of such salts include lithium tetrachloroaluminate and the solvents include those containing sulfur dioxide (SO.sub.2), thionyl chloride (SOCl.sub.2) and sulfuryl chloride (SO.sub.2 Cl.sub.2). An inert cathode collector such as one made of compressed carbon black completes the cell couple.
While the present invention is applicable to all non-aqueous cells that utilize active metal anodes with relatively small ions such as lithium or potassium it is particularly suited to cells designed to be anode-limited. An anode-limited cell may be defined as an active metal, non-aqueous electrochemical cell which, when discharged under the designed rates, will be depleted of anode material at levels .gtoreq.90% utilization when the cell reaches a voltage cut-off at .gtoreq.0.0 volts.
A basic problem with the electrochemical cells of the type described, especially those cells utilizing sulfur dioxide, thionyl chloride or sulfuryl chloride in combination with an active metal such as lithium, has been the problem of safe storage and operation of the batteries. Applications of such cells, are usually ones in which the cells are packaged in battery configurations in which a plurality of such cells are connected in series. It is well known that the discharge of a plurality of cells in such battery configurations may result in the possibility of cell voltage reversals due to overdischarge of one or more of the cells. Under this condition prior art batteries tend to experience undesirable reactions even at ambient temperature. This leads to pressure build up within the cell which may cause the cell to burst or, in severe cases, undergo a powerful detonation. This, of course, is quite undesirable, especially if the batteries are to be operated in the proximity of persons who might be physically harmed by such a turn of events.
Thus, a definite need remains to provide such cells with a means for preventing or reducing the hazards associated with voltage reversals. In accordance with the present invention, it has been found that the use of a ceramic separator possessing certain properties prevents voltage reversals in anode-limited cells of the class described.
In the prior art, several species of ceramic separators have been employed. One is illustrated and described in U.S. Pat. No. 4,283,469 to Goebel, et al in which a thin film of porous ceramic material, which may be sprayed in the form of a slurry, is applied to the cathode collector electrode to form a composite cathode collector. The only requirement of this film is that it be nonconducting in itself and be porous to the electrolyte solution. The function of the ceramic film is to prevent unwanted reactions involving lithium at high temperatures. In U.S. Pat. No. 4,407,910 to Catanzarite, as in Goebel et al, ceramic separators are utilized for their chemical properties of preventing reactions between highly reactive anodes such as lithium and other system components such as thionyl chloride at elevated temperatures.
None of the prior art cells either address or solve the problem of voltage reversal, however. The known prior art cells which utilize ceramic in any form between the anode and the cathode do so specifically to prevent high temperature lithium reactions which, although a definite hazard in such cells, represent a hazard quite different from that of voltage reversal. Voltage reversal is associated with overdischarge of anode-limited cells in which the lithium has been depleted or exhausted. Reactions between the lithium and other solution species either cannot occur or are of no consequence under this condition.